The present invention relates to a half-bridge circuit having a first and a second transistor that are connected in series, and having a drive circuit for driving the transistors. The invention relates, in particular, to a half-bridge circuit for use in a switching regulator.
The handbook xe2x80x9c1999/2000 Industrial Power Seminarxe2x80x9d, pages 6-3 and 6-7, from Intersil Corporation(trademark), Irvine, Calif. USA 92618, describes a switching regulator with a half-bridge circuit as shown in FIG. 1. The half-bridge circuit shown in FIG. 1 has two n-channel MOSFETs M1, M2, whose drain-source paths are connected to form a series circuit. This series circuit is connected to a supply voltage U1. A drive circuit 100 is provided for driving the MOSFETs M1, M2. The drive circuit 100 is connected to the gate connections of the MOSFETs M1, M2 and to the supply voltage for voltage supply purposes. So that the required drive potential for the first MOSFET M1, which functions as a high-side switch, can be made available in the drive circuit 100, a bootstrap circuit with a capacitor C2 and a diode D1 is provided, which is likewise connected to the drive circuit 100. The drive potential required for the first MOSFET M1 is higher than the supply voltage U1.
The known half-bridge circuit shown in FIG. 1 is part of a switching regulator, a so-called buck converter in the example illustrated, which provides a smaller output voltage U2 from the supply voltage U1. In this case, a series circuit including a coil L1 and a capacitor C1 is connected in parallel with the second MOSFET M2, which functions as a low-side switch. It is possible to tap off the output voltage U2 across the capacitor C1.
Switching regulators of this type are used, in particular, in computers for supplying voltage to the CPU. In this case, the input voltage is usually 5.0 V and the output voltage is between 1.3 V and 2.0 V. A customary clock frequency with which the two MOSFETs are switched is about 200 kHz. In this case, the two MOSFETs M1, M2 are driven, depending on the output voltage U2, such that the output voltage is at least approximately constantxe2x80x94independently of the load and of fluctuations in the input voltage.
For future applications in computers, the switching regulators ought to be able to correct a load change at the output of the switching regulator within a time period of less than 100 ns. That requires clock frequencies of 2 MHz or more for the switches.
In this case, the line connections between the components and the half-bridge should be as short as possible. Furthermore, the two transistors should be accommodated in one housing, as much as possible, to save space.
In the case of the known half-bridge circuit, a comparatively high outlay is required to accommodate the two transistors and possibly also the drive circuit. Vertically designed transistors are usually used as the power transistors in switching regulators. Such transistors are usually applied to a printed circuit board by their rear side, which forms the drain connection of the transistor. In the case of the known circuit arrangement shown in FIG. 1, different potentials are required at the drain connections of the transistors, so that a printed circuit board on which the two transistors are to be jointly applied must have at least two islands with different potentials. Moreover, if the intention is to accommodate the drive circuit on the same printed circuit board, at least three such potential islands must be provided.
It is accordingly an object of the invention to provide a half-bridge circuit which overcomes the above-mentioned disadvantages of the prior art apparatus of this general type.
In particular, it is an object of the invention to provide a half-bridge circuit for switching regulators, which can be realized in a space-saving manner and which can be used at comparatively high switching frequencies.
With the foregoing and other objects in view there is provided, in accordance with the invention, a half-bridge circuit including a first semiconductor body having a first MOS transistor that is integrated in the first semiconductor body. The first MOS transistor is a vertically designed n-conducting MOS transistor. The half-bridge circuit includes a second semiconductor body having a second MOS transistor that is integrated in the second semiconductor body. The second MOS transistor is a vertically designed p-conducting MOS transistor. The half-bridge circuit also includes: a drive circuit for driving the first MOS transistor and the second MOS transistor; a common connection plate to which the first MOS transistor and the second MOS transistor are applied; and a first connection terminal and a second connection terminal. The first MOS transistor and the second MOS transistor are connected in series between the first connection terminal and the second connection terminal.
In accordance with an added feature of the invention, the first semiconductor body has a front side and a rear side; the first MOS transistor has a drive connection and a first load path connection that are accessible at the front side of the first semiconductor body; the first MOS transistor has a second load path connection that is accessible at the rear side of the first semiconductor body; the second semiconductor body has a front side and a rear side; the second MOS transistor has a drive connection and a first load path connection that are accessible at the front side of the second semiconductor body; and the second MOS transistor has a second load path connection that is accessible at the rear side of the second semiconductor body.
In accordance with an additional feature of the invention, a capacitor is connected between the first load path connection of the first transistor and the first load path connection of the second transistor.
In accordance with another feature of the invention, the capacitor is applied to the first semiconductor body and to second semiconductor body; the capacitor has a first connection connected to the first semiconductor body; and the capacitor has a second connection connected to the second semiconductor body.
In accordance with a further feature of the invention, a bonding wire electrically connects the first connection of the capacitor to the first semiconductor body; and a bonding wire electrically connects the second connection of the capacitor to the second semiconductor body.
In accordance with yet an added feature of the invention, a first layer electrically connects the first connection of the capacitor to the first semiconductor body; and a second layer electrically connects the second connection of the capacitor to the second semiconductor body. The first layer and the second layer are made of either a soldering material or an electrically conductive adhesive.
In accordance with yet an additional feature of the invention, an electrically conductive adhesive solders or bonds the first semiconductor body and the second semiconductor body onto the common connection plate.
In accordance with yet another feature of the invention, the drive circuit is integrated into a third semiconductor body; the first semiconductor body has a front side; and the third semiconductor body is applied to the front side of the first semiconductor body.
In accordance with yet a further feature of the invention, a common housing surrounds the first semiconductor body, the second semiconductor body, and the drive circuit.
The invention makes use of the fact that in a half-bridge circuit with a p-conducting transistor and an n-conducting transistor, of which only one at a time is to be driven conductively, the drain connections of the two transistors are connected to one another and are thus at a common potential. The rear sides of the two semiconductor bodies in which the two transistors are integrated can thus be applied to a common electrically conductive connection plate. The respective drain connection is accessible at each of the rear sides. This connection plate is, in particular, a lead frame of a housing for integrated circuits. The source connections and the gate connections of the transistors are available for further interconnection at the respective front sides of the semiconductor bodies.
In the half-bridge circuit, the first p-conducting transistor and the second n-conducting transistor are connected in series between a first terminal for a first supply potential and a second terminal for a second supply potential. The integrated drain connections of the two transistors form the output of the half-bridge circuit in this case. In the case of the customary interconnection of the half-bridge circuit, a positive supply potential is present at the first connection terminal and a negative supply potential, or ground, is present at the second connection terminal. The p-channel transistor then functions as a high-side switch and the n-channel transistor functions as a low-side switch. Using a p-channel transistor as a high-side switch in the circuit arrangement provides the advantage that there is no need for a bootstrap circuit for providing an increased drive potential for the high-side switch.
The on resistance of a p-channel transistor is greater than that of an n-channel transistor having the same channel cross-sectional area. In the present invention, using a p-channel transistor does not constitute a disadvantage with regard to future expected applications. In future switching regulators, it is expected that the ratio of the input voltage to the output voltage will increase further. In present switching regulators with an input voltage of 5 V and an output voltage of about 1.2 V, this ratio is about 4.17. In switching regulators for future applications, in particular, in switching regulators for supplying voltage to a CPU in a computer, a ratio of the input voltage to the output voltage of 10 or more can be expected. In so-called buck converters, which provide a smaller output voltage from an input voltage, the relationship between the time duration during which a current flows via the high-side switch and the duration of a clock period is like the relationship between the output voltage and the input voltage. Given a ratio of input voltage to output voltage of 10, current flows through the p-channel transistor as the high-side switch only about 10% of the time, while a current flows via the low-side switch over 90% of the time. To better dissipate the heat loss to the outside, it is desirable for the power loss to be distributed uniformly between the two transistors. Since the low-side switch conducts nine times as long as the high-side switch, the resistance of the low-side switch is permitted to be only {fraction (1/9)} of the resistance of the high-side switch if the same power loss is obtained on both transistors. In other words: the resistance of the high-side switch is permitted to be nine times as large as the resistance of the low-side switch. The costs and the space requirement of a p-channel transistor which is used as high-side switch and whose on resistance is nine times as large as the on resistance of the associated n-channel transistor used as low-side switch are likely to be lower than those of the low-side switch.
Moreover, by dispensing with the bootstrap circuit, the half-bridge circuit can be realized in a space-saving and cost-effective manner. This holds true all the more as the capacitor of the bootstrap circuit is usually embodied as an external component, which particularly affects the costs in the case of conventional half-bridge circuits.
One embodiment of the invention provides for a capacitor to be connected between the source connection of the first transistor and the source connection of the second transistor, that is to say in parallel with the supply terminals. This capacitor serves for bridging switching spikes that can arise during the clocked driving of the two transistors. The capacitance of this capacitor can be comparatively small at high switching frequencies. Therefore, one embodiment of the invention provides for a first connection of this capacitor to be directly connected to the source connection at the front side of the first semiconductor body of the first transistor, and for the other connection of the capacitor to be connected directly to the source connection at the front side of the semiconductor body of the second transistor. The connection between the capacitor and the source connections is preferably effected by bonding wires or by directly soldering or bonding the capacitor connections onto the source connections. The capacitor is, in particular, a ceramic capacitor that has an elongate form and an exposed connection at each of the ends. An insulating ceramic layer covers the remainder of the capacitor.
A further embodiment of the invention provides for the drive circuit to be integrated in a third semiconductor body which is applied directly to the source connection at the front side of the semiconductor body of the n-conducting second transistor.
The two transistors, the drive circuit and, if appropriate, the capacitor of the half-bridge circuit can in this way be accommodated in a housing in a space-saving manner.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a half-bridge circuit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.